I have devised a scheme to cheat with the small green scantrons. Not sure if it works with the large grey ones yet.
Step 1) Pencil choice is crucial. I suggest Pentel, but I've heard most mechanical pencils work as well.
2) You must narrow your choices down to 2. Bubble in the one which you are the least sure about pressing down very hard. Now erase, dont erase too well though. Now bubble in the answer choice you want to go with by pressing down ever so lightly. The lead in these pencils appear almost as dark as the rest of your answer choices, however it is not in the paper as much, and the scantron machine should mark the erased choice.
3) Its a good idea to check the test as soon as you get it back, if possible before returning to your seat and let your prof know of any errors. And thus, the cheat is ended.
I found this out on accident and proceeded to manipulate it. It is very easy to use and abuse to your discretion. I'm not a scantron machine expert so I'm not sure if the grey scantrons are better about detecting this. More to come when I take a class that uses those.